While in Australia on a business trip I picked up a few electronics magazines (Silicon Chip). Inside was a simple PIC project which read the voltage from a standard lambda sensor and converted this into a Air/Fuel ratio, displaying it on a LED display. So I bought the kit from Jay Car and built it!
The meter appeared to draw too much current from the original sensor. When connected across the ECU lambda sensor the car seemed to run sluggish...removing the kit instantly cured it. Instead of wasting the kit I decided to fit a second lambda sensor into the exhaust and connect that. So the system is no completely independent of the ECU.
I used an unbranded sensor with the kit, this seems to work perfectly.
In order to insert a second sensor into the exhaust a bit of cutting and welding was needed. I enlisted the help of another Autobahnstormers member, Richard Brunt, who worked his exhaust magic again. With the new lambda positioned as close as possible to engine (near the OE sensor) it was wired up to the display. I used the three-wire sensor but in hindsight it would have probably been better to use the four-wire type which adds a separate sensor ground.
I've taken quite a bit of flack in using these sensors as a tuning aid. I've witnessed their use at a very prestigious engine building company (think touring cars) on their engine dyno to keep an eye on exhaust mixture. If used with caution I think you can set up an engine with reasonable accuracy, remembering it's better to be a bit rich if anything! I only wanted the device to allow me to set WOT mixtures. I have only gone "richer" than the original settings as breathing on my engine has been improved. This breathing improvement seems to have been confirmed by the meter. I had had to add 10% onto injector duration to achieve a reasonably constant 12 on the mixture display, with the original maps the display read 15.
When the engine is warm you can clearly witness the closed loop behaviour of the Motronic system, cycling up and down from 14.7:1 regularly. Surprisingly the cars keep in closed loop most of the time. It's only WOT (wide open throttle) and cold start which will cause open loop running. Even heavy part throttle will quickly be tied back to close loop and the air fuel ratio kept to 14.7:1
Another club member is working on a new circuit which will be connect to the original sensor and have a far higher impedance so it won't interfere with it's workings. When this is available I will post some information here.
DATASHEETS
Oxygen sensor output voltage analysis by Scott Ulen (converted from his original .xls spread sheet file with his blessing). Also covers it's response with respect to temperature and some scientific first principles. Worth a read.
LINKS
Bosch OEM and universal lambda sensors
The Australian magazine who published the kit instructions
Supplier of the kit.